How inlet tubing material affects the response time of water vapor concentration measurements

Measurements involving water in the vapor phase have to deal with the stickiness of the H₂O molecule: The associated adsorption and desorption processes can increase the response time of these measurements significantly. To achieve short response times in scientific instrument design, hydrophobic surface materials are used to reduce surface interactions in the tubing that guides the sample towards the analyzer. The study presented here focuses on the effects of the tubing material choice, length, humidity level, gas flow rate, and temperature on the observed response time. We use an Optical Feedback Cavity Enhanced Absorption Spectrometer (OFCEAS) designed for stable water isotope measurements at low water concentration (< 1000 ppm), which we connect to two bottles containing humidified synthetic air of different water concentration using 6.6-m tubing of different materials and surface treatments. Other parameters that are varied are the flow rate and the temperature of the tubing. With proper selection of tubing material and surface treatment, the contribution from the tubing to the overall response time for low water concentration isotopic measurements can be sufficiently suppressed for it to be neglected.